there some type of Wet friction plates, for either brakes, or power shift transmissions, or automatic transmission, (wet means they run in a hydraulic or transmission oil)
steels or disks of steel with teeth on them will fit between them and then a hydraulic piston will push against them putting pressure, and locking them together if enough pressure is applied, thus a clutch,
similar action is used as brakes,
the plates teeth lock in to one side, either the drum or the shaft and the steels lock in to to apposite,
and a piston when applied presses against them locking the shaft to the drum,
on an automatic transmission / power shift transmission, this is how the gear range is changed,
on brakes it locks a moving part to a stationary part, on clutches like in a dozer, it locks up the drive axles or releases them,
On some clutches there are planetary gears and they use a brake band, that stops the drumm from spinned and forces the rotary power to the out going shaft, in some transmission there is a combination of clutches that work the inside of the drum and some times the outside of the drum with a band, and depending on what clutches lock up is how the transmission shifts, or transmits power through the gears, to the drive axles,